An optical system is used in image duplication devices such as facsimile machines, copiers and printers. In general, the optical system is housed in a single housing unit and is located near an intermediate image-forming surface such as a photoreceptor drum. The optical system includes an image-forming light source, an image-reflecting surface and an image-focusing element to form a desired image on the intermediate or temporary image-forming surface by repeatedly scanning the image-forming light beam in a predetermined direction. To accomplish an efficient scanning, the image-reflecting surface has multiple reflecting surfaces and is rotated at a high speed. The light source emits an image-forming light beam and is located at a certain distance from the rotatable reflecting surface at a predetermined angle so that a desired scanning angle is obtained. For these and other reasons, the above described prior art optical housing unit generally takes a certain amount of space.
For color image duplication, the light source generally includes multiple image-forming beams each representing a color component such as cyan, magenta, yellow and black. Relevant prior art references illustrate certain spacial arrangements of the above described components in a multi-beam scanning system. For example, Japanese Patent Laid Open Application 4-127115 discloses that a set of the above described multiple image-forming beams is independently scanned by a common polygon mirror and that the reflected image-forming beams respectively form portions of a desired image via a set of image-focusing lenses.
Referring to FIG. 1, Japanese Patent Laid Open Application 2-250020 discloses that an optical scanning system includes a set of light sources LS1 through LS4, and each light source emits an image-forming light beam towards a common polygon mirror 107a. The light beams LB1 and LB2 are reflected and scanned by a first reflecting surface of the polygon mirror 107a while the light beams LB3 and LB4 are reflected and scanned by a second reflecting surface of the polygon mirror 107a. The reflected beams LB1 and LB2 are focused by a series of lenses 118a, 119a, 120a and 120b, and the focused images are then redirected downwardly by a pair of mirrors 121 and 122 for scanning the focused images in predetermined directions C1 and C2 on a respective temporary image-forming surface. Similarly, the reflected beams LB3 and LB4 are focused by a series of lenses 118b, 119b, 120c and 120d, and the focused images are then redirected downwardly by a pair of mirrors 123 and 124 for scanning the focused images in predetermined directions C3 and C4 on a respective temporary image-forming surface. A first group of cylindrical lenses 118a, 118b, 119a and 119b has refractive power only in a scanning direction as indicated in C1, C2, C3 or C4 while a second group of lenses 120a, 120b, 120c and 120d has refractive power in the scanning direction as well as in a sub-scanning direction which is perpendicular to the scanning direction. As a result, the desired image is initially formed behind or further away from the lenses and the polygon mirror.
Referring to FIG. 2, Japanese Patent Laid Open Application 2-250020 also discloses that an optical scanning system includes a set of light sources LS1 through LS4, and each light source emits an image-forming light beam towards a common polygon mirror 107b. The light beams LB1 through LB4 are reflected and scanned by a common reflecting surface of a polygon mirror 107b. The reflected beams LB1 through LB4 are focused by a series of lenses 128, 129, 130a, 130b, 130c and 130d, and the focused images are then redirected downwardly by a mirror 131, 132, 133 and 134 for scanning the images in predetermined directions D1, D2, D3 and D4 on a respective temporary image-forming surface. A first group of cylindrical lenses 128 and 129 has refractive power only in a scanning direction as indicated by arrows D1, D2, D3 and D4 while a second group of anamorphic lenses 130a, 130b, 130c and 130d has refractive power both in the scanning as well as sub-scanning directions. As a result, the desired images are initially formed behind or further away from the lenses and the polygon mirror. Since each image is formed further away from the polygon mirror, the above described prior art optical scanning system requires a certain overall dimension and fails to reduce its size.
In addition, the above arrangement also fails to reduce the polygon mirror size. Since the above described second groups of the vertically layered lenses require a certain distance between the centers of the lenses, the image-forming light beams also need to be separated at least by the same vertical distance in the neighborhood of 10 mm. Due to the minimal vertical distance between the light beams, the thickness of the polygon mirror cannot be reduced beyond the above described beam separation distance. The polygon mirror thickness is also positively related to a size of a motor to rotate the polygon mirror at a predetermined high rotational speed. In other words, the larger the polygon mirror is, the larger the motor size is.
In the efforts to manufacture compact duplication devices, the above described optical system needs to be reduced in size. The compact optical system may be achieved by reducing the optical scanning housing unit and or a polygon mirror.
Furthermore, in the efforts to maintain the accuracy of the scanning operation performed by the above described optical scanning system, the scanning operation must be monitored and adjusted so that a desired image is formed by the simultaneously emitted and processed multiple image-forming light beams.